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Torquing a crank bolt without a torque wrench.

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Torquing a crank bolt without a torque wrench.

Old 03-05-14, 05:25 PM
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WK95
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Torquing a crank bolt without a torque wrench.

I don't have a torque wrench so I'm going to have to rely on feel or ask my LBS to do it for me.

Ok, so Torque = Radius * Force

I weigh 150 lb which is about 676 Newtons. When using the wrench, I use press down at a point about 7 inches from the center of rotation which approximates to 0.1778 meters.

For the bottom bracket, the recommended torque is 50 - 70 Nm. For the crank, the recommended torque is 35-50 N*m .

Full body Weight - 7 inches/0.1778m from axis of rotation
T=676[N]*0.1778[m]= 120.[Nm]
Full body weight is obviously too much.

Full body Weight - 3.5[in]/0.0889[m] from axis of rotation
T=676[N]*0.1778[m]/2=60.1[Nm]
Much better for the bottom bracket as it's within range. Of course, I can't put my full body weight onto that without standing on the wrench which would be absurd.

Now I just need to know how much Force I can exert on a wrench when pushing down on it.

Anyone who can help me out with this will be greatly appreciated.
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Old 03-05-14, 05:27 PM
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Get a torque wrench and a hex set for it.. you'll be glad you did. Or go to the LBS and have them do it, guestimating nm's and whatnot isn't going to work very well.. especially since we can't definitively say how much force you put out with your arms and that wrench.
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Old 03-05-14, 05:33 PM
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Ok, I suppose I'll take it down to my LBS then.

And here I was, thinking I could put my college physics skills to use...
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Old 03-05-14, 06:10 PM
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I like your approach, and would like to hear responses from other DIY types who can advise. Sorry I can't, but I like your idea of choosing a point along the lever arm that gives the appropriate torque from your bodyweight. Maybe localize the pressure by putting a rope around the handle at the desired radius and leaning on the rope?
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Old 03-05-14, 06:20 PM
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by all means try it, hate to see a good physics education going to waste. OTOH, i use the 1 grunt, 2 grunt, or occasionally the 3 grunt metric...
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Old 03-05-14, 06:24 PM
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Sidenote: If you're in wine country you can bring your bike on over and we can can figure out your output using my Torque wrench
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Old 03-05-14, 06:27 PM
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OK, this is silly. Going to a shop is fine. For those who can't do that just use a 7-8 inch wrench and put as much force as you reasonably can on it by hand (not standing on it - close enough.

If you are concerned about getting at least roughly the right amount of torque here's a diy solution off the top of my head. Do the calculation for what lbs/kgs of pressure is needed for the length of wrench you have (using the center of your hand on the wrench as the distance). Push down on a bathroom scale a few times until you have a sense of how that pressure feels. Loosen the bolt then tighten to that same feel.
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Old 03-05-14, 06:38 PM
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Originally Posted by WK95 View Post
Ok, I suppose I'll take it down to my LBS then.

And here I was, thinking I could put my college physics skills to use...
Two good reasons torque wrenches exist:
1. The force must be tangent to the radius (perpendicular to the wrench).
2. Torque values for threaded fasteners assume dynamic torque. The fastener must be turning at the moment the desired value is reached.
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Old 03-05-14, 06:54 PM
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Originally Posted by RaleighSport View Post
Sidenote: If you're in wine country you can bring your bike on over and we can can figure out your output using my Torque wrench
I shall begin planning for my cross country expedition from the East coast to visit your humble abode so that I may partake in your wisdom and borrow your torque wrench.
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Old 03-05-14, 06:56 PM
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Originally Posted by cny-bikeman View Post
OK, this is silly. Going to a shop is fine. For those who can't do that just use a 7-8 inch wrench and put as much force as you reasonably can on it by hand (not standing on it - close enough.

If you are concerned about getting at least roughly the right amount of torque here's a diy solution off the top of my head. Do the calculation for what lbs/kgs of pressure is needed for the length of wrench you have (using the center of your hand on the wrench as the distance). Push down on a bathroom scale a few times until you have a sense of how that pressure feels. Loosen the bolt then tighten to that same feel.
Good idea about using a bathroom scale. Unfortunately, I am not pro enough to have either a torque wrench or even a bathroom scale.
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Old 03-05-14, 09:10 PM
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Get an inexpensive beam type from the auto supply store.
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Old 03-06-14, 09:08 AM
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Admittedly, the use of a simple torque wrench is the easiest method to employ here however, in the event a torque wrench is unavailable, the following method allows the required leverage point on a lever arm to be determined, at which point an individual’s full body weight can be applied to the lever arm (while the lever arm is perpendicular to gravity’s vertical influence) as measured from the crank arm’s axis of rotation to produce the desired torque.

Scenario:

Suppose we don’t have access to a torque wrench, but we’re required to apply 50 Nm (Newton meters) of torque to tighten the crank arm bolts to their max 50 Nm torque rating. We’ll need to determine the precise point on a lever arm at which to apply our full body weight of 150 pounds to produce the required torque. What distance from the crank arm’s axis of rotation is required to accomplish this task?

Solution:


Start by converting your body weight “in pounds” to a body weight as measured “in Newtons” and this is easily accomplished as follows:

Newtons = pounds / .224961492

Hence,

150 pounds / .224961492 = 666.780784 Newtons of body weight

Next, we apply the desired 50 Nm of torque and our 666.780784 Newtons of body weight as follows:

50 Nm / 666.780784 Newtons = .074987164 meters

We’ve now determined the precise distance (.074987164 meters) away from the crank arm’s axis of rotation that our 666.780784 Newton body weight must be applied on the lever arm in order to produce the desired 50 Nm of torque.

If you need to convert the derived lever arm distance from meters to inches, simply divide the distance yielded “in meters” by the mathematical constant of .0254 as shown below:

.074987164 meters / .0254 = 2.952250551 inches (which is just several 64[SUP]ths[/SUP] of an inch short of 3 inches)

And so, the point on the lever arm at which we must apply our full body weight of 666.780784 Newtons is just a tad less than 2 and 61/64 inches away from the crank arm’s axis of rotation.

We can verify the accuracy of our derived leverage point by merely applying it via the following long established physics equation:

T = F * r

Whereby,

Torque “in Nm”= Force “in Newtons” * radius “in meters”

Hence,

666.780784 Newtons * .074987164 meters = 50 Nm of torque

As demonstrated, the precise lever arm length was derived by these straightforward calculations.

For those interested in the precise conversion factor to convert Nm of torque into foot pounds of torque:

Nm of torque * .738062638 = foot pounds of torque

50 Nm
* .738062638 = 36.90313189 foot pounds of torque

Though it isn’t necessary to use 10 digits worth of calculator precision, I prefer to do so in order to acquire precision outcomes that can afterward be rounded to however many digits of precision is deemed satisfactory.
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Old 03-06-14, 09:14 AM
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Originally Posted by WK95 View Post
Ok, I suppose I'll take it down to my LBS then.

And here I was, thinking I could put my college physics skills to use...
you can. get a good job with the degree, then use the money to buy some tools
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Old 03-06-14, 09:47 AM
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Originally Posted by Gnosis View Post
(snip rehash of original post). . . Though it isn’t necessary to use 10 digits worth of calculator precision, I prefer to do so in order to acquire precision outcomes that can afterward be rounded to however many digits of precision is deemed satisfactory.
Too bad significance does not work that way. Arithmetic can be dangerous in the wrong hands. FYI, force is a vector.
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Old 03-06-14, 10:12 AM
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After a few decades of using a square taper crank .. and Shop work ...


A crank bolt wrench will work , just that over the web forum, ' how tight' is more easily transmitted

by using a standard unit of measure , and to have that the scale on a torque wrench is useful .
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Old 03-06-14, 11:44 AM
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The nut on the end of the axel on my old VW van required 300' lbs of torque. My 1/2" wrench didn't register that high but at about 150lbs, I snugged it up by standing on the end of a 2' breaker bar. It was probably not accurate to several decimal points but I drove it another severeal years...
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Old 03-06-14, 01:16 PM
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over tightening the crank bolt in square taper cranks stretches the aluminum and ruins the crank arm.
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Old 03-06-14, 01:24 PM
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Harbor Freight $10 on sale right now.

Good to well under the 4% it is guaranteed to be. I've checked mine often and it is always under 2% error.

If you really can't trust a $10-20 tool then spend another $30 and check the two against each other. It's super-easy. As long as the two agree then you are Good to Go.
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Old 03-06-14, 01:41 PM
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Originally Posted by fietsbob View Post
over tightening the crank bolt in square taper cranks stretches the aluminum and ruins the crank arm.
My bottom bracket and crank use Octalink V2 so it has splines. Based on the design of Octalink, I wouldn't expect ovrtorquing to have as serious a consequence. I still don't want to take any chances on that though.
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Old 03-06-14, 02:02 PM
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Originally Posted by cny-bikeman View Post
OK, this is silly. Going to a shop is fine. For those who can't do that just use a 7-8 inch wrench and put as much force as you reasonably can on it by hand (not standing on it - close enough.

If you are concerned about getting at least roughly the right amount of torque here's a diy solution off the top of my head. Do the calculation for what lbs/kgs of pressure is needed for the length of wrench you have (using the center of your hand on the wrench as the distance). Push down on a bathroom scale a few times until you have a sense of how that pressure feels. Loosen the bolt then tighten to that same feel.
Or....stand on an analog batroom scale while tightening the bolt and stop when the scale has reached the desired value. Would that work?
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Old 03-06-14, 02:05 PM
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My bottom bracket and crank use Octalink V2
Witholding that data as to what crank/bb you had
did not advance your desire for specific answers much .

back to my previous statement .

..just that over the web forum, ' how tight' is more easily transmitted
by using a standard unit of measure , and to have that the scale on a torque wrench is useful .
so go to the Shimano technical site , and seek out the torque spec number..


40 years as a bike mechanic I have the feel for sufficient ,
but my hands can only type words and numbers over the Net.

Buy the tools , hire the work done , or just obsess over something else.

Last edited by fietsbob; 03-06-14 at 02:12 PM.
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Old 03-06-14, 02:18 PM
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Originally Posted by SquidPuppet View Post
Or....stand on an analog batroom scale while tightening the bolt and stop when the scale has reached the desired value. Would that work?
Problem is determining lever length, and I would think it would be easiest to push down on the wrench, which means you would check for the amount of decrease in the weight. .
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Old 03-06-14, 02:41 PM
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DIY doesn't mean doing it without the proper tools. Nashbar sells a nice torque wrench kit. Should last a lifetime.
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Old 03-06-14, 02:49 PM
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Originally Posted by SquidPuppet View Post
Or....stand on an analog batroom scale while tightening the bolt and stop when the scale has reached the desired value. Would that work?
Sort of the opposite.
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Old 03-06-14, 02:53 PM
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Sort of the opposite.
yea your weight is reduced,and transferred to the wrench handle ,
so the % of your weight on the scale would be less..

then the length of the wrench handle in feet or inches or Meters
and your weight on the end of that handle in pounds, Kg.

deduced by how much less is on the Loo Scale ..

Last edited by fietsbob; 03-06-14 at 02:57 PM.
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